1. Field of the Invention
This invention relates to a melt-blown non-woven fabric having a wide fiber diameter distribution, in which fine fibers with different diameters are adequately dispersed, and also to a nozzle piece applied for producing the same.
2. Prior Art
Production of non-woven fabrics by melt blowing has been known since a long time. The principle was disclosed by, e.g., U.S. Naval Research Laboratory's Report No. 5256, Feb. 11, 1959. It was also described in detail by EXXON Chemical, which commercialized the production for the first time in the world, in Japanese Laid-open Patent Application No. 50-46972 and "Melt Blowing-One-step Web Process for New Non-woven Products (Tappi Journal, vol. 56, No.4, April, 1973).
In this process, a number of nozzles of a given diameter is disposed at a given "hole-to-hole" pitch in front of the die, and molten polymer discharged therefrom is spun in a flow of a hot air jet, wherein a non-woven fabric is produced with fine fibers having a relatively uniform diameter.
The diameter of the fibers is optionally controlled by process conditions of melt blowing, the major ones including polymer temperature, and rates of discharged polymer and air. The non-woven fabric thus prepared is characterized by a relatively uniform diameter of the fibers which constitute the fabric.
When such a non-woven fabric is applied to, e.g., a liquid filter, filtration efficiency is largely determined by the diameter of the fibers which constitute the fabric.
However, a filter composed of very fine fibers tends to be clogged in a short time with the particles and such clogging decreases service life, although it can capture very fine particles.
So far melt-blown non-woven fabrics composed of fiber of different diameters have been developed, in order to solve the above problems. For instance, they are generally produced by the multi-die process which uses two or more dies equipped with nozzles of different sizes respectively, or by lamination in which two or more non-woven fabrics each composed of fibers having different diameters are placed one on another, in order to obtain the required filtration efficiency of the filter and extend its service life. The lamination to each other is performed by using an adhesive or by heat with thermal calender rolls.
These methods, however, tend to increase production cost, and also require delicate processing control. Such additional process may cause quality-related problems, e.g., deterioration of the fabrics by heat or delamination during the service period.